rc-local - How to enable ???

[JD]

# systemctl enable rc-local
The unit files have no [Install] section. They are not meant to be enabled
using systemctl.
Possible reasons for having this kind of units are:
1) A unit may be statically enabled by being symlinked from another unit's
    .wants/ or .requires/ directory.
2) A unit's purpose may be to act as a helper for some other unit which has
    a requirement dependency on it.
3) A unit may be started when needed via activation (socket, path, timer,
    D-Bus, udev, scripted systemctl call, ...).

So, what does a user do in order to make the system start (execute)
/etc/rc.d/rc.local at (or shortly thereafter) boot up??